The increasing demand for electrical energy requires continuous development of improved energy storage technologies. Batteries store and release energy via chemical reactions, but have limited storage capabilities. While batteries can achieve high energy density values, discharge rates are typically limited by the chemical reactions. In contrast, ultracapacitors do not rely on chemical reactions to release energy. As such, ultracapacitors, also referred to as supercapacitors, can be charged and discharged rapidly.
Ultracapacitors, also called supercapacitors or electrochemical capacitors, are a potential solution for meeting the world's electrical energy storage needs. Vastly accelerated adoption of ultracapacitor technology, now mainly based on porous carbons, is currently limited by the low energy storage density and relatively high effective series resistance of these materials.
Ultracapacitors store energy by forming a double layer of electrolyte ions on the surface of conductive electrodes. Ultracapacitors are not limited by the electrochemical charge transfer kinetics of batteries and thus can operate at very high charge and discharge rates, and can have lifetimes of over a million cycles; however, the energy stored in ultracapacitors is currently an order of magnitude lower than batteries. The limited energy storage of ultracapacitors limits their use to those applications that require high cycle life and power density. The energy density of conventional state-of-the-art ultracapacitor devices, mainly based on porous activated carbon (AC), is about 4-5 Wh/Kg while that of lead acid batteries is in the range 26-34 Wh/Kg. A conventional AC material, with a specific surface area (SSA) in the range of 1,000-2,000 m2/g and a pore size distribution in the range of 2-5 nm, can have a gravimetric capacitance of 100-120 F/g in an organic electrolyte.
Significant research has thus been focused on increasing energy density without sacrificing cycle life or high power density. Ultrathin, high surface area carbon films including graphene and graphene-like materials have been identified as promising candidates for use as ultracapacitor electrodes. However, it has been observed that the storage capacity of these materials in ultracapacitors is intrinsically limited. For example, activated microwave expanded graphite oxide (‘aMEGO’) materials having high surface area and energy density have been recently described. These materials, however, may exhibit saturation of capacitance at lower than desirable levels. Similarly, recent investigations of “pristine” monolayer graphene have shown that the area-normalized charge storage of suspended monolayer graphene that can be stored simultaneously on both sides of a graphene monolayer is significantly lower than could be stored on a single side of a graphene monolayer.
Thus, there is a need to address the problems and other shortcomings associated with existing ultracapacitor technology and carbon materials for use therein. These needs and other needs are satisfied by the compositions and methods of the present disclosure.